SVI can consume masses of HDD space if not controlled, e.g. see limits on its size. You can also clean it out, by setting a new restore point and then clearing old restore points.
Create Clean Restore Point - Clear old Restore Points.
If your system is running fine and you are clear of infection create a clean System Restore point:
1. Click Start, All Programs, Accessories, System tools, System Restore.
2. In the pop-up that appears fill in the radio button to Create a Restore Point
3. Click NEXT
4. Enter a useful name that you will remember if you need to find this again (Clean Restore Point)
5. Click CREATE
You now have a clean restore point, you should clear the old ones:
1. Click Start, All Programs, Accessories, System tools, Disk Clean Up
2. Click OK on the C: drive
3. Click the More Options tab
4. In the System Restore section click the Clean Up button
Item 1. in both may be different for Vista so I leave you to check that out.
Note, if you have multiple partitions there will be an SVI on each.